We seek to identify enzyme modifications which lead to the increased synthesis of lactic acid in neoplastic cells. Using a control and a transformed epithelial cell line, we identified the following modifications with neoplastic transformation: There are marked alterations in the kinetic properties of lactate dehydrogenase (LDH) which can account for the ten-fold increase in lactic acid produced in the transformed cells. This is accompanied by molecular modifications in the subtypes of LDH in the transformed cell. We are purifying LDH in order to determine the structural differences between subtypes from the two cell lines. Following exposure to alkaline phosphatase, molecular subtypes of LDH in the control cell are altered to give the appearance of the transformed type. Further investigation of this is under way with (serine-) protein kinase. The membrane transport properties of the transformed cell appear to be unaltered with respect to lactate, suggesting that the increased output of the acid in the transformed cell is due to the new kinetic properties of LDH. Since cells from the two lines have the same appearance by light microscopy, even though one produces carcinomas in vivo, we are examining the cytoskeletal proteins. Results show a normal alignment of tubulin in the transformed cell. The two cell lines do differ in ultrastructure, the transformed cell being very deficient in mitochondria. This may also serve to enhance the output of lactic acid through the incomplete oxidation of substrates. Thus both the glycolytic and mitochondrial system show modifications which can serve as points of attack in neoplastic cells.